Positioning a mobile device, such as a smart phone, in an indoor venue may be useful in many use scenarios. For example, merchants in an indoor mall may send targeted marketing information to nearby mobile devices based on the position information of the mobile devices. As another example, the position information of mobile devices in an indoor venue may greatly aid search and rescue missions.
Time-of-Arrival (TOA) measurements-based trilateration techniques may be utilized to determine the position of a mobile device. A plurality of local transmitters whose locations are known may transmit positioning beacon signals including information identifying the transmitting local transmitter and indicating the time of transmission. Much like the way the Global Positioning System (GPS) works, a receiver device, such as a mobile device, may determine its position based on TOA measurements of the received positioning beacon signals and the known positions of the local transmitters using trilateration techniques.
Unlike some other types of positioning systems such as a Round-Trip-Time (RTT) measurements-based positioning system, a TOA-based positioning system has the advantage of being scalable. In other words, because the local transmitters in a TOA-based positioning system merely broadcast signals indicating their identity and the time of transmission, an increase in the number of receiver devices does not increase the workload of the local transmitters. Thus, a TOA-based positioning system may theoretically support an unlimited number of receiver devices.
For a TOA-based positioning system to work, the clocks of the local transmitters need to be almost perfectly synchronized. It has been suggested that the increasingly ubiquitous wireless local area network (WLAN) access points (e.g., wireless network devices that provide wireless network service in accordance with one or more IEEE 802.11 protocols on the industrial, scientific, and medical “ISM” radio frequency bands) may double as local transmitters for indoor positioning applications. The clock of a conventional WLAN access point is, however, generally of a poor quality. The clocks of a plurality of WLAN access points are unlikely to remain synchronized for long without very frequent resynchronization.